The present invention relates to a rotary screw machine having at least one pair of rotors which are affected by forces from the working fluid in a first axial direction, at least one of the rotors being provided with shaft journals supported by bearing means, including main thrust bearing means and thrust balancing bearing means having a rotating ring and a stationary ring and being provided with thrust balancing means, said thrust balancing means including spring means acting on said stationary ring in said first direction and fluid pressure means acting axially on said stationary ring.
A compressor of this kind is known from U.S. Pat. No. 3,388,854. In that disclosure, the stationary ring of the thrust balancing bearing is pre-loaded by a spring 35 acting in the same direction as the axial gas forces on the rotors. The stationary ring also abuts a fluid actuated piston 36, through which a force can be applied to the ring counteracting the force from the spring. The pressure chamber 37, through which a pressure can be applied to the piston 36 can be connected to a pressure source, i.e. the outlet channel. This is the case under normal operating conditions, whereby the thrust load will be distributed to the thrust bearings on both ends of the rotor. When the compressor is idling, the pressure chamber is relieved to the atmosphere so that only the spring pre-loads the stationary ring. Through this known device, in many cases, a satisfactory distribution of the axial forces on the thrust bearings is attained, but it entails still some limitations regarding an optimal distribution of these forces.
The object of the present invention is to improve the known thrust balancing device in order to reach such a force distribution on the thrust bearings so that the resultant force on each thrust bearing falls within a more narrow range, thereby increasing the possibility to meet the requirements for a sufficient working life for each of the thrust bearings.